Traditional indoor temperature control, such as implemented in a water carried heating system, is based on a simple relationship between the outdoor and the forward heat carrier temperature. Typically the controller is provided with a number of preset curves, often composed of one or several lines with specified slope, offset, and possibly intersection points. The user then selects a curve that corresponds to the characteristics of the building. Optimization is normally done by trial and error during a calibration period.
A problem with such calibration is that a building hardly ever is in equilibrium, with the environment (the weather variations are normally faster than the time constants of the building). Therefore, it is typically very difficult to determine a suitable slope and offset.
In recent years, it has been suggested to model the heat transfer between the building and the environment, and use this model in the control process. On example of this is described by the Swedish Metrology and Hydrology Institute (SMHI) and marketed by Honeywell under the name Weathergain. According to this system, the house model, and predictions of the future climate variations, are used to convert the current outdoor temperature into an “equivalent” outdoor temperature. This “equivalent” outdoor temperature is then supplied to a traditional control system, making it slightly more efficient.
Another example is given in U.S. Pat. No. 6,439,469, where a model of the house heat exchange and weather predictions are used for a predictive control of flow temperature.
Document WO 91/16576 describes a method for controlling a heat generating appliance by monitoring flow and return temperatures of the heated fluid, the flow rate, room temperature and external ambient temperature.
Document U.S. Pat. No. 4,089,462 describes a temperature control system arranged to shift the control point in accordance with the ambient temperature and the K-factor of the room in which the system is operating.